1. Field of the Invention (Technical Field)
The present invention relates to compositions and methods of making and using same as an adsorbent or catalyst material, particularly activated carbons.
2. Background Art
Activated carbon finds widespread use for adsorption of gaseous, liquid and dissolved materials. Typically such materials are present as toxins or contaminants in a fluid or process stream; however, sometimes the adsorbed material has value. For instance, many metal ions pose both a risk (heavy metal toxicity) and a benefit (resale value). In the photographic film processing industry, the economics of silver recovery are favorable if silver ions can be efficiently recovered from wastewater. In addition, less silver flows into the environment. Most removal processes make use of an activated carbon filter, e.g., cartridges, fluidized beds, packed beds, etc., through which the process stream flows. As the process stream flows through the filter, the contaminant or other material is adsorbed onto the surface of the activated carbon. Activated carbon derives its adsorptive properties predominantly from a high surface area to mass ratio. The adsorbed materials essentially condense on the activated carbon's solid surface. The so-called carbon "activation" process, of which there are many, enhances the carbon's surface area to mass ratio.
Commonly used activation processes treat the carbon containing raw material stock in a thermodynamic and/or chemical manner. Thermodynamic treatments includes high temperature and high pressure processes while chemical treatments typically rely on acids or bases like phosphoric acid or sodium hydroxide. Some chemical activation processes use Lewis acids like zinc chloride. In most instances, thermal treatment processes use temperatures exceeding 500.degree. C., thereby making energy input an important economic consideration. The raw materials may come from synthetic or natural sources, e.g., resin wastes, coal, coal coke, petroleum coke, lignites, polymeric materials, and lignocellulosic materials including pulp and paper, residues from pulp production, wood, nut shell, kernel and fruit pits. Organic materials find widespread use as a starting material; however, supply issues may impact availability. Often pre-treatment steps prepare the raw material for activation. For instance, fracturing materials like nut shells and fruit pits through grinding increases the raw material's available surface area thereby increasing the effectiveness of the activation step(s). An increase in surface area will typically decrease resistance to both heat and mass transfer. A decreased resistance to mass transfer facilitates chemical permeation into the interior of the raw material while a decreased resistance to heat transfer facilitates both heat diffusion and conduction into the interior of the raw material. Whether through thermodynamic and/or chemical means, the activation process enhances the pore structure and leads to a significant increase in the surface area to mass ratio.
Typical commercially available activated carbon products have a specific surface area to mass ratio of at least 300 m.sup.2 g.sup.-1 while some have ratios exceeding 2000 m.sup.2 g.sup.-1. Much of the surface area resides, however, within the activated carbon's porous structure. Therefore, the activated carbon's pore size distribution and tortuosity may control selectivity as well as the rate of adsorption. Some commercially available activated carbon products have pore widths less than 10 .ANG.; small pore widths generally exclude large molecules. Activated carbon with a pore size distribution skewed toward small widths may not function effectively to remove large molecules, such as dyes, from waste streams. In general, most commercially available activated carbon products do not function effectively to remove dyes from waste streams. Other commercially available activated carbon products fail to effectively remove ionic species. Ionic species often have hydrophilic properties and, in aqueous solutions, carry a hydrated shell of substantial thickness. Thus, activated carbon that possesses hydrophobic surface characteristics may not adsorb ionic species effectively.
Another issue that arises with most commercially available activated carbon is deactivation, a process whereby the adsorptive capacity of the carbon decreases through use. The two main options to overcome deactivation are replacement and reactivation through regeneration. Processes used for regeneration often mirror those used for activation. Most existing regeneration methods require treatment of the "deactivated" carbon in an oven at high temperatures. First, however, the "deactivated" carbon must be removed from the process, few techniques are capable of in-situ regeneration. Second, absent a low cost energy source, the economics of heat driven regeneration are seldom favorable since the carbon usually experiences a drop in effectiveness after several high temperature regeneration cycles. The present invention provides a composition and method of making same that is both useful for removing large molecules and ionic molecules with an additional advantage in that the carbon can be regenerated in-situ, without removal from the filtration system. The present invention also uses temperatures that minimize energy input and are thus economically favorable.
Prior art related to the invention includes U.S. Pat. No. 5,710,092, to Baker, entitled "Highly Microporous Carbon," (discloses activation of a carbon material at temperatures from 650.degree. C. to about 1100.degree. C.); U.S. Pat. No. 5,416,056, to Baker, entitled "Production of Highly Microporous Activated Carbon Products," (discloses a two step combined chemical and thermal process for activation of lignocelluloic material wherein the first step uses temperatures between 150.degree. C. and 590.degree. C. and the second step uses temperatures between 650.degree. C. and 980.degree. C.); U.S. Pat. No. 5,407,574, to Hensley, entitled "Filter Media for Filter Systems" (employs mixture including crushed pecan hulls as filter media); U.S. Pat. No. 5,356,852, to DeLiso et al., entitled "Activated Carbon Structures" (uses activated carbon as a starting material for making filters thereby avoiding need for firing/sintering in making the filters); U.S. Pat. No. 5,198,398, to van Duijn, entitled "Method for Regenerating Spent Activated Carbon and Portable Container for Use Therein" (regeneration of activated carbon at temperatures between 800.degree. C. and 1000.degree. C.); U.S. Pat. No. 5,102,855, to Greinke et al., entitled "Process for Producing High Surface Area Activated Carbons," (discloses a process using thermal treatment of carbon at temperatures from 450.degree. C. to 1200.degree. C.); U.S. Pat. No. 5,039,691, to Kosaka et al., entitled "Chemically Activated Shaped Carbon, Process for Producing Same and Use Thereof," (discloses a process that utilizes temperatures between 500.degree. C. and 700.degree. C. for activation of carbon material); U.S. Pat. No. 4,760,046, to Burger et al., entitled "Process for the Production of Activated Carbons using Phosphoric Acid," (discloses a two step combined chemical and thermal process for activation of carbon wherein the first step uses a rapid thermal treatment between 80.degree. C. and 250.degree. C. followed by a step using temperatures between 250.degree. C. and 500.degree. C.); U.S. Pat. No. 4,643,182, to Klein, entitled "Disposable Protective Mask" (gas-adsorbing activated carbons, including from pecan nut shells, employed); U.S. Pat. No. 4,616,001, to Sato, entitled "Activated Carbon," (activates macadamia nut shells at temperatures between 400.degree. C. and 900.degree. C.); U.S. Pat. No. 4,569,756, to Klein, entitled "Water Treatment System" (activated carbons, including from pecan nut shells, used in water treatment); U.S. Pat. No. 4,454,044, to Klein, entitled "Water Treatment Process" (activated carbons, including from pecan nut shells, used in water treatment); U.S. Pat. No. 4,395,332, to Klein, entitled "Adsorption and Filtration Mat for Liquids" (activated carbon, including from pecan nut shells, employed); U.S. Pat. No. 4,344,775, to Klein, entitled "Method for Separating Gases Liquid Droplets and/or Solid Particles from Gases or Vapors" (activated carbon, including from pecan nut shells, employed); and U.S. Pat. No. 4,239,516, to Klein, entitled "Porous Media to Separate Gases Liquid Droplets and/or Solid Particles from Gases or Vapors and Coalesce Entrained Droplets" (activated carbon, including from pecan nut shells, employed).